Ralph comes over to the Cunningham house to tell everyone "I just enlisted in the United States Marines." After he leaves, Richie says Ralph has "gone to sign up (enlist)," even though Ralph just told them a few minutes ago he enlisted. And later on after that, Potsie gives the news to the gang that Ralph's dad wouldn't let Ralph enlist, so Ralph is running away to Chicago to enlist.
Mr. C is seen reading a copy of "Lizards as Pets" by Richard Headstrom. This book wasn't published until 1971.